Within the spectrum of polymeric compounds that contain diphenylsiloxane units, oligomers are known, inter alia, in the form of cyclic oligomers, siloxanes stopped at both terminals by the trimethylsilyl group, and siloxanes bearing silanol at both terminals. Examples of the cyclic oligomers are hexaphenylcyclotrisiloxane, which is the diphenylsiloxane trimer, and octaphenylcyclotetrasiloxane, which is the tetramer. These are diphenylsiloxane compounds that are fairly soluble in organic solvents and melt at between room temperature and 300.degree. C. However, they lack chemical reactivity because they are composed of the chemically stable phenyl group and the similarly stable siloxy bond. In addition, the corresponding solids are in essence organic crystals, and as such their material strength is quite low. In consequence thereof, their exploitation as materials, either alone or in combination with other materials, is unpromising.
With regard to diphenylsiloxane oligomer capped at both terminals with trimethylsilyl, 1,1,1,9,9,9-hexamethyl-3,3,5,5,7,7-hexaphenylpentasiloxane is known from Journal of the Chemical Society of Japan, Industrial Chemistry Section, Volume 62, page 1421 (1959). This is also a soluble and fusible diphenylsiloxane. But again, it is composed of the chemically stable phenyl group and the similarly stable trimethylsiloxy group and thus is extremely stable and very inert.
Diphenylsiloxane bearing silanol at both terminals is exemplified by tetraphenyldisiloxane-1,3-diol and hexaphenyltrisiloxane-1,5-diol. These, too, are soluble and fusible diphenylsiloxanes, which again are composed of the chemically stable phenyl group and the similarly stable siloxy bond. Only the silanol group may be classified as a functional group capable of readily participating in bonding. The silanol group is in general able to participate in the formulation of compositions through the formation of hydrogen bonds the functional groups in another substance or through bond formation by reaction with other hydrolyzable groups. However, when bonded to the diphenylsiloxy group, the silanol group is very inert. In addition, at high temperatures the silanol group participates in a "backbiting" reaction that is characteristic of polysiloxanes bearing this functional group. Stability then becomes a problem due to the resulting depolymerization reaction.
Moving up to the level of polymers comprising diphenylsiloxane units, the polydiphenylsiloxane homopolymers are already known. These can be prepared by the ring-opening polymerization of hexaphenylcyclotrisiloxane. These homopolymers are brittle white crystalline solids that undergo a phase transition in the vicinity of 260.degree. C. to yield a mesophase. This mesophase exhibits poor fluidity, which makes the molding and processing of these homopolymers quite difficult. While these homopolymers could be expected to exhibit satisfactory fluiditics at temperatures above their melting points, they melt at 500.degree. C. and above, at which temperatures they undergo thermal decomposition. Thus, rather severe problems are associated with the processing of these polymers and the formation of compositions with other materials.
The polydimethylsiloxanes are the most typical siloxanes and have various types of derivatives. Many derivatives with the structures given in formula (A) below are known. Oligomers of this class generally remain liquid even at temperatures several tens of degrees Centigrade below the freezing point and thus cannot be hot-melt molded. The polydimethylsiloxane derivatives are prepared by an equilibration reaction between cyclic polydimethylsiloxane oligomer and an end group generally known as an end blocker. However, in the case of dipenylsiloxane, the equilibrium between the cyclic oligomer and straight-chain polymer is strongly skewed toward production of the cyclic oligomer. As a result, diphenylsiloxane oligomer functionalized at both terminals (hereinafter abbreviated as diterminal-functionalized diphenylsiloxane oligomer) essentially cannot be prepared by the analogous preparative method, i.e., using an equilibration reaction. ##STR2##
In addition to the preceding, siloxane structures described by the following formula (B) have been widely reported (for example, in Japanese Patent Application Laid Open Number Hei 5-32783). These are siloxane structures that carry various types of functional groups at the two terminals of a diphenylsiloxane-dimethylsiloxane random copolymer. ##STR3## wherein m and n are integers, A is a functional group bonded to the silicon across a divalent organic group and pH herein after denotes a phenyl radical. The main chain in these polymers has a random copolymer structure, which results in these polymers being soluble in solvents and exhibiting fluidity when heated. These polymers, therefor, differ in their composition, properties, and preparation from the diterminal-functionalized diphenylsiloxane oligomers introduced and disclosed by the present invention. In particular, the nature of the contribution of the siloxane fraction to the physical properties of compositions is substantially different for these siloxanes, both when used alone and when used in combination with other materials.